The present invention relates to a semiconductor optical amplifier characteristic evaluation method and apparatus and, more particularly, to a characteristic evaluation method for a semiconductor optical amplifier (to be referred to as an SOA hereinafter) used in an optical transmission system for optical communication, optical switching, optical signal processing, or the like.
An optical transmission system using light for optical communication, optical switching, optical signal processing, or the like uses various kinds of optical devices. In such an optical transmission system, an optical loss poses a serious problem, and therefore, it is indispensable to compensate for an attenuated optical signal using optical amplifiers. Of optical amplifiers essential in an optical transmission system, an SOA is compact and highly efficient. A semiconductor optical amplifier is very promising because it can be hybrid-integrated with a planar lightwave circuit (PLC) constituted by a quartz-based optical waveguide.
To use SOAs in optical transmission systems, they must undergo chip evaluation to screen non-defective units before hybrid mounting or module mounting.
For such screening, an SOA characteristic evaluation apparatus as shown in FIG. 10 is conventionally used. FIG. 10 shows the schematic arrangement of the characteristic evaluation apparatus. As shown in FIG. 10, the conventional SOA characteristic evaluation apparatus comprises a multi-wavelength optical source 101 for emitting multi-wavelength light, a wavelength selection switch 105 for selecting a specific wavelength from the multi-wavelength light emitted from the multi-wavelength optical source 101, an optical attenuator 106 for adjusting the optical intensity, a polarization controller 102 for generating various polarized states, optical power meters 104 and 108 for monitoring output light (amplified spontaneous emission or ASE) from an SOA 115, an optical circulator 103, a coupler 107, a wavelength filter 109 for screening a wavelength, and an optical power meter 110 for measuring the optical intensity. These components are connected using optical fibers.
To evaluate the characteristic of the SOA using the apparatus shown in FIG. 10, first, optical fibers 113 and 114 are aligned to the two ends of the SOA 115 while monitoring, by the optical power meters 104 and 108, output light (amplified spontaneous emission or ASE) from the SOA 115 to be evaluated.
After the optical fibers 113 and 114 are aligned, the wavelength selection switch 105 selects a specific wavelength from multi-wavelength light emitted from the multi-wavelength optical source 101. The optical attenuator 106 adjusts the optical intensity. The polarization controller 102 controls the polarized state. The light is thus guided to an end face of the SOA 115 through the optical fiber 113. The external light is thus input to the SOA 115.
On the other hand, the optical power meter 110 measures the optical intensity of input/output light from the SOA 115, which is guided to the wavelength filter 109 through the optical fiber 114 and undergoes noise component removal by the wavelength filter 109.
In this state, the polarization controller 102 generates various polarized states. The absolute value of the gain of the SOA 115 can be calculated from the measurement value by the optical power meter 110 at this time. In addition, the dependence of the gain on polarization can be measured by measuring the maximum or minimum value of the optical intensity by the optical power meter 110.
For semiconductor lasers having a structure similar to an SOA, the elements are evaluated on the basis of the current vs. optical output characteristic (I-L characteristic) by pulse driving in order to remove the influence of heat.
For the SOA 115, however, since light is input to it through the optical fiber 113, the amplified optical intensity of the external injection light decreases due to the fiber coupling loss at the two ends of the SOA 115. To ensure a sufficient measurement sensitivity in the above-described evaluation of the SOA 115, CW current driving is executed.
However, for CW current driving of the chip of the SOA 115, the chip of the SOA 115 must be mounted on a heat sink that is excellent in heat dissipation. Since the evaluation chip must be bonded, it cannot be mounted in a product.
In addition, to evaluate the characteristic of the SOA 115, its two ends must accurately be coupled to fibers. To do this, a fine optical fiber alignment is needed at the two ends of the SOA 115. Furthermore, precise alignment on the submicron order is necessary. Measuring the gain by the procedure for both end facets of the SOA takes a lot of time.
Hence, in the above-described conventional evaluation method, chip evaluation is executed by sampling inspection. The characteristics of chips to be used for products are unknown.
Additionally, the SOA 115 has a broad gain bandwidth. Hence, to accurately evaluate the characteristic of the SOA 115, the dependence of the gain on wavelength must be measured.
The SOA 115 must also be evaluated to check how much the gain characteristic changes depending on the polarized state of incident light (the dependence of the gain on polarization). The number of evaluation items is larger than that of a semiconductor laser, and therefore, the inspection is time-consuming.
The present invention has been made to solve the above problems, and has as its object to make it possible to continuously and accurately evaluate the characteristics of all manufactured semiconductor optical amplifiers (chips) in a short time with an arrangement more inexpensive and simpler than before by, e.g., executing characteristic evaluation such as characteristic screening of elements to be used for products without using any fiber coupling.
In order to achieve the above object, according to the present invention, there is provided a semiconductor optical amplifier characteristic evaluation method comprising supplying the current to a semiconductor optical amplifier, measuring an optical output generated by the semiconductor optical amplifier that has received the current, measuring transmission light obtained by transmitting the optical output through optical transmission adjustment means, and evaluating a characteristic of the semiconductor optical amplifier on the basis of a measurement result of the optical output and a measurement result of the transmission light without using an optical input to the semiconductor optical amplifier.